For many years a link in the chain of Samuel Butler's biological works has been missing. "Unconscious Memory" was originally published thirty years ago, but for fully half that period it has been out of print, owing to the destruction of a large number of the unbound sheets in a fire at the premises of the printers some years ago. The present reprint comes, I think, at a peculiarly fortunate moment, since the attention of the general public has of late been drawn to Butler's biological theories in a marked manner by several distinguished men of science, notably by Dr. Francis Darwin, who, in his presidential address to the British Association in 1908, quoted from the translation of Hering's address on "Memory as a Universal Function of Original Matter," which Butler incorporated into "Unconscious Memory," and spoke in the highest terms of Butler himself.